GDand T reference chart PDF

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Download GDand T reference chart PDF

Description

GD&T REFERENCE GUIDE SYMBOL

NAME

FOR SURFACE TOLERANCE ZONE OR F.O.S.? SHAPE (see below)

CAN USE MMC OR LMC MODIFIER?

CREATES VIRTUAL CONDITION?

DATUM REF?

Straightness

Either

a, b, c

Yes, if a F.O.S.

Never

Yes, if on F.O.S.

Flatness

Either

b

Yes, if a F.O.S.

Never

Yes, if on F.O.S.

Circularity

Surface

d

No

Never

No

Cylindricity

Surface

e

No

Never

No

Profile of a Line

Surface

a, d, f

No*

Usually

No

Profile of a Surface

Surface

b, e, g

No*

Usually

No

Parallelism

Either

a, b, c

Yes, if a F.O.S.

Always

Yes, if on F.O.S.

Perpendicularity

Either

a, b, c

Yes, if a F.O.S.

Always

Yes, if on F.O.S.

Angularity

Either

a, b, c

Yes, if a F.O.S.

Always

Yes, if on F.O.S.

Position

F.O.S.

b, c, h

Yes

Always †

Yes

Concentricity

F.O.S.

c

No

Always

Yes

Symmetry

F.O.S.

b

No

Always

Yes

Circular Runout

Surface

d

No

Always

Yes**

Total Runout

Surface

e

No

Always

Yes**

Form controls

Profile controls

Orientation controls

Location controls

Runout controls

a = 2 parallel lines

b = 2 parallel planes

e = 2 coaxial cylinders

c = cylinder

d = 2 coaxial circles

f = 2 irregular line boundaries

g = 2 irregular surface boundaries

h = sphere

*The datum reference(s) may use MMC or LMC modifiers, if a F.O.S. datum †

Exception: position applied to coaxial holes

**It is not a true "virtual condition" as defined in the standard, but it does create a "worst-case boundary" that may be important for assembly or function.

F.O.S. stands for “feature of size” (a pin, hole, slot, etc.)

Helpful things to remember:

The current standard for GD&T is ASME Y14.5-2009, from the American Society of Mechanical Engineers. It replaces the 1994 version. The rectangular box that contains a GD&T callout is known as the "feature control frame." A geometric tolerance shown in a feature control frame is always total, not plus/minus. Depending on how it is used, it may be centered around a fixed location, or it may float within a given size limit. The datum references (the letters at the end of a feature control frame) are given in a specific order to show the relative importance of each (primary, secondary, and tertiary).They do not have to be in alphabetical order, but rather order of precedence. The modifier M is helpful for clearance fits. It allows the tolerance to increase as the size of the feature varies. It can also be used on datum references if there might be looseness or "play" on those features. Datum features should be identified on physical items (surface, hole, pin, etc.) not on an imaginary center line. Even if the true datum might be a center, the symbol should still appear on the feature from which the center is derived. Basic dimensions (boxed dimensions) do not have any direct tolerance. Instead, they are indirectly toleranced from a feature control frame. Basic dimensions are most common in conjunction with position and profile controls. Concentricity is expensive to inspect. Often, position or runout can be used to achieve the same goal. (Reason: concentricity measures the centers of every cross-section, but position measures the center of an envelope, and runout measures the physical surface.) One of the most powerful GD&T symbols is profile of a surface. It controls a shape (which is defined by basic dimensions) by building a three-dimensional tolerance zone around it. And depending on how it relates to the datums, it can also control orientation and location. Modifiers to know:

M = maximum material condition P = projected tolerance zone

©2009 Geometric Learning Systems

L = least material condition U = unequal tolerance zone

F = free state T = tangent plane www.gdtseminars.com...